Gray's Reef National Marine Sanctuary
Gray's Reef National Marine Sanctuary
Gray's Reef Expedition 2012
Mission Information
 

Friday: June 1, 2012
Log Day 2

Dr. Laura Kracker
Geographer
NOAA / NOS / NCCOS

Laura runs fisheries acoustic sonar surveys to map fish at GRNMS

Laura runs fisheries acoustic sonar surveys to map fish at GRNMS
(Photo: Debbie Meeks)

Here I am again, very happy to be back with the Gray's Reef team on their annual survey onboard the NOAA Ship Nancy Foster. The Sanctuary is a special place and I appreciate the fact that it is a diverse yet 'compact' study area. It makes me think that perhaps we can understand how this little bit of ocean works. While Gray's Reef Sanctuary is a small rectangle when shown on a map of the southeast coast of the US there are large forces affecting it - like Tropical Storm Beryl that just passed through. It is tempting to think of the Sanctuary as a homogenous expanse of water, especially when you conduct surveys from the surface like I do. However, survey tools that use sound in water reveal the diverse habitats and fish communities below the surface.

Transducer mounted on the hull of the ship while in drydock

Transducers mounted on the hull of the ship while in drydock.
Click here for larger view
(Photo: Laura Kracker)

The Nancy Foster is equipped with fisheries echosounders - basically fancy fish finders - mounted to the hull of the ship to allow us to quickly map the distribution of fish over a large area. These surveys use the properties of sound in water to help us explore and map fish and other organisms in the entire water column. We can use this information to answer questions such as where do fish like to hang out and how abundant are they?

Acoustic survey results are displayed on an echogram. The red line is the seafloor. The top of the echogram is the just below the ship. Here a large school of fish about 5m off the bottom (center) passed under the ship at 5:30 am.

Acoustic survey results are displayed on an echogram. The red line is the seafloor. The top of the echogram is the just below the ship. Here a large school of fish about 5m off the bottom (center) passed under the ship at 5:30 am.
Click here for a larger view
(Photo: Laura Kracker)

Acoustic fisheries survey techniques involve emitting a pulse of sound or ping into the water and then 'listening' for the echo return reflected off objects in the water column and the bottom. We measure the strength of this echo return to give us an estimate of how large a fish is (the louder the echo return, the larger the fish). We measure the time that it takes to 'hear' the echo return to determine the depth of individual fish in the water column (the longer it takes for the echo to return, the deeper the fish). Combining this information with GPS data, we can tell the geographic position, depth, and size of any fish we pass over on our survey. This information is displayed on an echogram which shows fish targets and fish schools.

Schooling pattern on this image indicates this might be spadefish

Schooling pattern on this image indicates this might be spadefish.
(Photo: Laura Kracker)

There are several ways that we use data from fisheries surveys on this mission. First, we can map fish at the same time that we map the bottom using the multibeam system. So whenever we are mapping new areas in or around the Sanctuary, we also map fish to discover what seafloor features are out there and if fish are abundant in these newly mapped areas. We also conduct fish acoustic surveys at ledge sites where our dive teams are collecting data. Divers descend to the seafloor during the daytime to count and identify fish. At dawn and dusk, we conduct fisheries sonar surveys at these same sites to get a wider view of fish distribution around the dive sites. Whereas divers cover up to 50 meters in a single 30 minute dive, the ship travels 1000 meters every five to six minutes while collecting acoustic fish data. The data that the divers collect help us identify the fish targets that we are seeing on our sonar surveys. Likewise, the sonar surveys provide the divers and Sanctuary managers a broader view of how fish are distributed around the dive sites and across the entire Sanctuary.

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